The present invention relates to the field of duplex communication transceivers, and more particularly to FM duplex transceivers which utilize dual conversion receiver circuitry wherein the received signal is frequency converted to a first IF signal, then frequency converted to a second IF signal, and then demodulated.
In duplex communication transceivers, the transmitter and receiver portions of the transceiver are both simultaneously operative. In some of these systems the difference between the frequency of the transmit carrier signal (the carrier signal to be radiated) and the frequency of the receive carrier signal equals the first IF (intermediate frequency) signal frequency, since the difference between the transmit and receive carrier frequencies equals the channel spacing between the transmit and receive signals. In addition, some of these transceivers utilize dual conversion receiver circuitry in order to obtain the benefits thereof wherein this circuitry involves the utilization of first and second mixers for converting the received carrier signal to a first IF signal and then to a second IF signal, respectively.
In FM (frequency modulation) duplex communication transceivers such as those described above, typically the first injection signal supplied to the receiver first mixer is provided by a first local oscillator and comprises a fixed frequency signal. In some transceivers, the transmit carrier signal and the receiver first mixer injection signal are related since the fixed frequency receiver injection signal is developed first and then the modulated transmit carrier signal is produced through the use of a sidestep transmit mixer which mixes the receiver first injection signal with an offset signal, modulated by a transmit information signal, which is developed by an offset oscillator. While such transceivers provide for having the receive and transmit carrier frequencies track each other, thus enabling the transceiver to be readily tuned to any of a number of different channels, these systems typically require the utilization of an offset oscillator and sidestep mixer and therefore are relatively costly. This cost disadvantage essentially occurs in each such prior transceiver where the modulated transmit signal is not produced by direct modulation of an oscillator providing as its output the transmit carrier signal or a subharmonic thereof. Even in transceivers where the frequency difference between the transmit and receive carriers is the IF frequency, these transceivers require an additional transmit modulation stage if the transmit carrier is first generated and used as the receiver first injection signal and the transmit carrier is then modulated by the transmit information signal. It is more cost effective to directly modulate the transmit carrier oscillator than to provide a fixed frequency transmit carrier signal and then provide an additional modulation stage. Thus in prior transceivers such as those discussed above either additional mixers and/or modulators and/or oscillators are utilized in order to provide both the modulated transmit carrier signal to be radiated and the first injection signal used for the receiver first mixer.
The present invention deals with a technique for utilization of the modulated transmit signal as the first receiver injection signal for the first mixer. In such systems there is a problem of cancellation of the transmit modulation in the receiver circuitry. One prior system uses costly additional filtering and an additional IF transmitter mixer such that the modulated transmit signal, after filtering removes the transmit modulation, can be used as the receiver first injection signal. Obviously this solution is not cost effective since it requires additional mixer and filter stages and it also creates additional transmitter spurious signals. In a copending U.S. patent application, Ser. No. 332,432, filed Dec. 21, 1981 to Kurby, entitled, "Radio-Frequency Synthesizer for Duplex Radios", and assigned to the same assignee as the present invention, another type of cancellation solution is proposed. However, that solution is not cost effective since it utilizes an additional offset oscillator and sidestep mixer in order to provide the transmit carrier signal which is modulated by the transmit information signal. In both of these prior systems the signal actually used as the receiver first injection signal is an unmodulated constant frequency signal since the transmit modulation has been previously removed from the receiver first injection signal either by filtering or by cancellation.
In a prior doppler radar system a transceiver is disclosed in which the modulated transmit signal is used directly as a receiver first injection signal, but in this system typically the receive and transmit carriers have identical frequencies, except for any doppler frequency shift. Also in this system the end result is not demodulation of the modulating receive signal information, which is identical to the transmit modulation signal information, but the end result is the production of the doppler frequency shift signal. In other words, the transmit carrier and its modulation signal are used to cancel the receive carrier signal and its modulation, and thereby isolate the doppler signal. Thus obtaining the received modulating signal is not obtained by and is not an object of this prior radar system, and this is contrary to most communication transceivers, including those of the present invention.